The present invention generally relates to communication terminals, and more particularly to a communication terminal which controls power transfer and is suited for application to an integrated services digital network (ISDN) terminal.
An ISDN provides a plurality of different communication services such as telephone, data, facsimile and image communications by use of a communication network including a digital switching system and digital transmission lines. In the ISDN, when a power down occurs on the network (switching system) side, restricted power with a polarity which is inverted from a normal power is supplied from the network side.
FIG. 1 shows an example of a conventional ISDN. A plurality of ISDN terminals TE are coupled to an ISDN switching system EX via network terminator devices (or digital service units) NT1 and NT2. The network terminator device NT1 and NT2 include functions for making subscriber's line termination, layer 1 maintenance, monitoring, power transfer, layer 1 multiplexing, interface termination, protocol processes of layers 2 and 3, layer 2 multiplexing, layer 3 multiplexing, and interface termination.
FIG. 2 shows an example of a conventional ISDN terminal. A transmission signal line 100 is coupled to a frame disassembly/assembly part 105 of a line interface part 104 via a transformer T1 and drivers 102. A receiving signal line 101 is coupled to the frame disassembly/assembly part 105 of the line interface part 104 via a transformer T2 and receivers 103. The frame disassembly/assembly part 105 disassembles/assembles the B1 and B2 channels and the D channel within 2B+D. The D channel is transmitted and received between a central processing unit (CPU) interface 106 of the line interface part 104 and each of a CPU 108, a random access memory (RAM) 109 and a read only memory (ROM) 110 via a data bus 107. The B1 and B2 channels are transmitted and received between a B channel selector 111 of the line interface part 104 and a handset 115 via a coder/decoder 112, a sending amplifier 113 and a receiving amplifier 114. A dialing part 119 is connected to the CPU 108.
On the other hand, the signal lines 100 and 101 are coupled to a power input line 117 via a diode bridge 116, and the power input line 117 is coupled to internal circuits via a power sink 118 and a power input line 219. The power is supplied to the power sink 118 regardless of whether or not the ISDN terminal is carrying out a communication.
For example, there are cases where a plurality of terminals having no local power supply are connected to the communication network via the switching system and cases where the local power supply is down. In such cases, when the plurality of terminals simultaneously require the power supply from the switching system side, the switching system in conformance with a communication standard can only supply 400 mW per subscriber's line. As a result, there are problems in that the load on the network side increases and the terminal side cannot operate in some cases. In other words, according to this communication standard, one terminal is only supplied with a power of 380 mW from the switching system side so that the remaining 20 mW can be supplied to other terminals to enable at least a terminal to detect a call. However, since the terminal supplied with the power of 380 mW constantly operates a detection circuit for detecting the call, considerable power is consumed by the detection circuit and the power supplied to internal circuits of this terminal is reduced thereby. Therefore, there is a demand to fully utilize the limited power supplied to the terminal from the switching system side.